1. Field of the Invention
This invention relates to levelling of heavy steel plates and especially to apparatus and methods for roller levelling of steel plates having a yield strength up to about 250,000 psi and thicknesses from about 3/16 inch to about 1/2 inch.
2. Prior Art
The roller levelling of heavy gauge, high strength metal plate is a very difficult operation, and typically has required the use of extremely heavy and expensive levelling equipment. Heretofore, in conventional roller levelling, in order to effectively level thick, high strength metal plate, it has been necessary either to pass the plate back and forth between the levelling rollers or to provide a plurality of roller stands through which the plate is successively passed and in each of which successive stands, the deflection between opposed upper and lower work rolls is decreased so that there is little or no bending of the plate material exiting the last such stand. In the first case, multiple reversing passes of the plate through the leveller requires upstream space at least equal to the length of the plate being levelled. In the latter case, multiple stands also require substantial space and are very expensive so as to be impractical.
Numerous prior art devices are known comprising two or more clusters of opposed upper and lower work rolls for performing two or more physical treatments of sheet or strip, such as correction of edge waves (where the edges of the sheet or strip are longer than the center portion); "oil canning" (where a center portion of the sheet or strip is longer than the edges) ; correction of strip "crown"; and flattening of the sheet or strip of widely varying types of products, such as relatively thin sheet and strip and heavier gauge plate wherein different roll diameters are used to level materials of different thicknesses. Examples of such prior art devices include: Blough U.S. Pat. No. 4,633,697; Thompson et al. U.S. Pat. No. 3,623,348; Thompson et al. U.S. Pat. No. 3,638,326; Roesch U.S. Pat. No. 3,701,274; Schlueter U.S. Pat. No. 3,606,784: Klempay U.S. Pat. No. 3,466,913; Maust U.S. Pat. No. 2,963,070; Maust U.S. Pat. No. 2,945,530, and Japanese Patent Publication No. 60-171526 (A).